Due to the accomplishment of catching more than three billion Pokémon during the first-ever Pokémon GO Global Global Catch Challenge, the Legendary Ho-Oh is now appearing around the world until December 12. The official Ho-Oh trailer and announcement from the development team at Niantic is as follows:
— Pokémon GO (@PokemonGoApp) November 27, 2017
Congratulations! We issued a challenge—and you answered the call. Collectively, you caught over 3 billion Pokémon during the Global Catch Challenge, unlocking some amazing rewards along the way. But that’s not all… As a result of all your hard work, Ho-Oh is now appearing in Raid Battles at Gyms around the world until December 12, 2017. So, gather your friends and find a Legendary Raid near you!
—The Pokémon GO team
Source: Official Pokémon GO blog